Many drugs are known for the treatment of osteoarthritis, but in general their effectiveness is low, especially if side-effects are to be avoided. A known drug of this type is diclofenac sodium.
In normal articular cartilage (human as well as murine), monoamine oxidase in chondrocytes can be found located precisely in mitochondria. During the development of the natural disease in mice, these granules become larger and apparently burst, the monoamine oxidase (MAO) activity becomes distributed through the cytoplasm of the chondrocytes and even into the surrounding matrix. This was described by Chambers et al., Int. J. Exp. Path. 73:115-123 (1992), who related the MAO activity in the medial as against that in the (normal) lateral cartilage in untreated mice and in those treated with diclofenac.
The presumption from these findings was as follows: when circulating pharmacologically-active amines, such as adrenalin, enter normal chondrocytes they are detoxified by the MAO inside the mitochondria. Such detoxification requires both the close association of the MAO with the coenzyme FAD (flavine adenine dinucleotide) and the close presence of a peroxidase to remove the H.sub.2 O.sub.2 generated by the MAO activity. When the MAO occurs in the matrix of the cartilage, it is unlikely to be functional, because (i) it may have been disrupted from its coenzyme (FAD), and (ii) if it were to be operative, it would be generating H.sub.2 O.sub.2 which would be damaging to the matrix.
When diclofenac sodium was given to mice, there was a marked improvement in the localisation of the MAO in the potentially osteoarthritic cartilage. There was no sign of activity in the general cytoplasm or in the matrix.
It was suggested that the diclofenac molecule might become split to yield monoamines which would be a weak substrate for the relevant enzyme, and act as competitive inhibitors of the MAO activity. However, diclofenac sodium did not stop the development of OA in these mice.
Diclofenac sodium is related to fenamic acid. Derivatives of fenamic acid have been used as anti-inflammatory agents, particularly influencing the cyclooxygenase and/or 5-lipoxygenase systems; see U.S. Pat. No. 5,114,958.